.box{
    position: relative;
    width: 100%;
    height: 39px;
}

.left,
.right{
    width: 20px;
    height: 39px;
    background: url(../images/bg02.png);
    background-position: 0px -120px;
    position: absolute;
    right: 0px;
}

.right{
    /* background-position: -20px -120px; */
    background-position-x: -20px;
    /* 
        background-position === background-position-x + background-position-y 两个参数的集合
    */
    right: unset;
    left: 0px;
}